The present invention relates to an isoprene-butadiene copolymer rubber having improved processing properties.
Methods for producing a butadiene polymer or a styrene-butadiene copolymer having a high content of 1,4-trans linkage are already known in the art. These polymers can be produced by solution polymerization in the presence of a complex initiator comprising as the main components (1) an organic compound of a metal from Group IIA and (2) another organic compound of a metal. [See U.S. Pat. No. 4,080,492 and B.P. 1,525,381.] According to the description in these patents, the distinctive feature of the polybutadiene polymer and the styrene-butadiene copolymer having a high content of trans-1,4 linkage obtained by such polymerization methods is a high green strength, and these polymers are said to exhibit the same high green strength and elongation as natural rubber.
These polymers have the excellent features of possessing high green strength but, on the other hand, possess such defects as having a poor tackiness and bad mill behavior. One of the ways to remove such defects is to widen the molecular weight distribution of the polymers by certain methods. [See Japanese Patent Application O.P.I. 52-9090, or its U.S. counterpart, U.S. Pat. No. 3,992,561.] However, in this case, defects are introduced by widening the molecular weight distribution, that is, a deterioration of the rubber elasticity, especially the resilience thereof can be seen. In an effort to overcome the above-described defects, it has been discovered that by using an isoprene as a portion of the monomer for copolymerization, the tackiness and the processing properties of the polymer can be improved, while the molecular weight distribution of the polymer is kept comparatively narrower.